Conventional ultrasound scanners are capable of operating in different imaging modes, such as B mode and color flow mode. In the B mode, two-dimensional images are generated in which the brightness of a display pixel is based on the intensity of the echo return. In color flow imaging, the flow of blood or movement of tissue can be imaged. If movement is present in the object being imaged, a Doppler shift in the return signal (relative to the transmitted signal) is produced which is proportional to the speed of movement.
Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receive ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers such as blood cells. This frequency, i.e., phase shift, translates into the velocity of the blood flow. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate.
The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. The velocity may be displayed using different colors to represent magnitude and direction. The total power in the Doppler spectrum can also be displayed. Color flow images are produced by superimposing a color image of the velocity or power of moving material, such as blood, over a black and white anatomical B-mode image. Typically, color flow mode displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image and color-coded to represent each sample volume's velocity or power.
In a conventional ultrasound imaging system (shown in FIG. 1), an ultrasound transducer array 2 is activated to transmit a series of multi-cycle (typically 4 to 8 cycles) tone bursts which are focused at the same transmit focal position with the same transmit characteristics. These tone bursts are fired at a pulse repetition frequency (PRF). The PRF is typically in the kilohertz range. A series of transmit firings focused at the same transmit focal position are referred to as a "packet". Each transmit beam propagates through the object being scanned and is reflected by ultrasound scatterers such as blood cells. The return signals are detected by the elements of the transducer array and then formed into a receive beam by a beamformer 4. The beamformer sums the delayed channels data and outputs either RF or in-phase and quadrature (I/Q) data. The latter alternative is illustrated in FIG. 1. In the conventional system, the frequencies of the beamformer outputs are filtered by a filter 6 and then output, depending on the operating mode, either to a B-mode processor 8 or to a color flow processor, comprising a pair of wall filters 12 and a parameter estimator 14.
In the B mode, an envelope detector incorporated in B-mode processor 8 forms the envelope of the beam-summed receive signal by computing the quantity (I.sup.2 +Q.sup.2).sup.1/2. The envelope of the signal undergoes some additional B-mode processing, such as edge enhancement and logarithmic compression, to form display data which is output to the scan converter 16.
In the color flow mode, the I/Q components are output to a corner turner memory 10, the purpose of which is to buffer data from possibly interleaved firings and output the data as vectors of points across firings at a given range cell. Data is received in "fast time", or sequentially down range (along a vector) for each firing. The output of the corner turner memory is reordered into "slow time", or sequentially by firing for each range cell. The resultant "slow time" I/Q signal samples are output to the color flow processor, which incorporates respective wall filters 12 that reject any clutter corresponding to stationary or very slow-moving tissue.
Given the angle .theta. between the insonifying beam and the flow axis, the magnitude of the velocity vector can be determined by the standard Doppler equation: EQU v=cf.sub.d /(2f.sub.0 cos .theta.) (1)
where c is the speed of sound in blood, f.sub.0 is the transmit frequency and f.sub.d is the motion-induced Doppler frequency shift in the backscattered ultrasound.
The wall-filtered outputs are fed into a parameter estimator 14, which converts the range cell information into the intermediate autocorrelation parameters N, D, and R(0). N and D are the numerator and denominator for the autocorrelation equation, as shown below: ##EQU1## where I.sub.i and Q.sub.i are the input data for firing i, and M is the number of firings in the packet. R(0) is approximated as a finite sum over the number of firings in a packet, as follows: ##EQU2## R(0) indicates the power in the returned ultrasound echoes.
A processor in parameter estimator 14 converts N and D into a magnitude and phase for each range cell. The equations used are as follows: ##EQU3## The parameter estimator 14 processes the magnitude and phase values into estimates of power, velocity and turbulence. The phase is used to calculate the mean Doppler frequency, which is proportional to the velocity as shown below; R(0) and .vertline.R(T).vertline. (magnitude) are used to estimate the turbulence (variance).
The mean Doppler frequency is obtained from the phase of N and D and the pulse repetition time T: ##EQU4## The mean velocity is calculated using the Doppler shift equation: ##EQU5## The parameter estimator 14 does not calculate the mean Doppler frequency as an intermediate output, but calculates v directly from the phase output of a processor using a lookup table.
In general, the display data input to the scan converter 16 is in R-.theta. format (for a sector scan), which is converted by the scan converter into X-Y format for video display. The scan-converted frames are passed to a video processor 18, which maps the video data to a grey scale and/or color mapping. The grey scale and/or color image frames are then sent to the video monitor 20 for display. Typically, either velocity or power are displayed alone or velocity is displayed in conjunction with either power or turbulence. System control is centered in a host computer (not shown in FIG. 1), which accepts operator inputs through an operator interface (e.g., a keyboard) and in turn controls the various subsystems.
If the ultrasound probe is swept over an area of interest, two-dimensional images may be accumulated to form a three-dimensional data volume. The data in this volume may be manipulated in a number of ways, including volume rendering and surface projections. In particular, three-dimensional images of B-mode (intensity), velocity or power data can be formed by projecting maximum, minimum, average or median pixel values onto an imaging plane.
Three-dimensional reconstructions of B-mode data suffer because the data is difficult to segment due to the lack of contrast and dynamic range. Reconstructions of velocity and power data suffer because associated clutter signals cause errors in segmentation. Thus there is a need for a three-dimensional imaging technique having improved segmentation.